The present invention relates to the fabrication of printed circuits, and more particularly to the provision in printed circuits of selected readily-solderable surfaces (e.g., on pads, through-holes and lands) for facilitating the subsequent connection thereat, by soldering, of electronic components.
In the manufacture of electronic equipment utilizing printed circuits, connections of electrical components to the printed circuit are made, e.g., by soldering of the leads of the components to the through-holes and surrounding pads provided on the printed circuit and/or by soldering of electronic components directly to surface areas ("lands") provided on the printed circuit (for so-called surface-mounted devices or SMD's), typically by wave soldering techniques.
To facilitate this soldering operation, the printed circuit fabricator is required to arrange that the through-holes, pads and lands are receptive to the solder connection process, i.e., possess surfaces which are wettable by the solder and which will permit an integral conductive connection, via the solder, with the leads or surfaces of the electronic components.
The primary means for arranging this good solderability of the surfaces in question is for the printed circuit fabricator to provide these surfaces with a solder coating, keeping in mind that at the same time other surfaces of the printed circuit (e.g., conductive traces) are generally to be masked by the fabricator so that, in the subsequent wave soldering of components to the printed circuit, these surfaces will not adherently receive solder. Provision of solder on these throughhole, pad or land surfaces is relatively straightforward because, in a typical printed circuit fabrication process, a tin-lead layer will in any event be selectively electroplated onto all exposed copper circuitry areas, including traces, pads, through-holes and lands, to serve as an etch-resist in the subsequent steps wherein copper is etched away at the selected non-circuitry areas of the printed circuit. Thereafter, a solder mask can be selectively applied to areas other than the holes, pads and lands, and the electroplated tin-lead can then be reflowed and fused to form the requisite solder alloy on these connection areas.
For a variety of reasons, the foregoing process is not ideal. In particular, the reflow of the tin-lead to form solder at the desired exposed holes, pads and lands, and the subsequent wave soldering operations, also cause the tin-lead under the solder mask at trace areas to become molten. The solder mask literally floats on the molten tin-lead layer, and wicking of the tin-lead up beneath the mask can result in formation of undesirable solder bridges between traces and proximate conductive areas on the circuit.
One means for avoiding these problems is the so-called "solder mask over bare copper" (SMOBC) technique, wherein there is no tin-lead on the copper traces which are covered by the solder mask. This technique eliminates the problems of wicking and solder bridging, but is generally expensive due to added processing steps. Thus, in the earlier-described typical fabrication process, the selectively plated tin-lead used as an etch resist must thereafter be stripped from the underlying copper areas, solder mask then applied over the areas (including now bare copper traces) other than pads, holes and lands, and then solder applied over the unmasked copper pads, holes and lands. Certain SMOBC techniques are known which avoid the need for tin-lead stripping, but these also involve additional processing steps.
In processes of this type, i.e., wherein it is necessary for the fabricator to apply solder to pads, holes and lands on a printed circuit other than by reflow of a tin-lead layer already present thereon by virtue of the particular requirements of a printed circuit fabrication process, a number of solder application possibilities exist, including use of non-electrolytic tin-lead immersion baths into which the printed circuit is immersed (after solder masking of areas not to be plated) to provide a tin-lead coating which can then be reflowed. By far the most prevalent method, however, is by hot air solder levelling, in which the masked printed circuit is immersed in a molten solder and, upon removal, hot air knives then used to blow the solder off all non-adherent surfaces. Hot air solder levelling provides satisfactory solderable pads, holes and lands, for subsequent connections of electronic components thereto via wave soldering, but a number of disadvantages also exist, not the least of which is the severe thermal stress placed on the printed circuit as a consequence of the 200.degree. C.+ molten solder treatment, often leading to pull-away of metal in through-holes and other like problems.